Mysql add foreign key constraint fails - mysql

Question

I'm trying to add a foreign key constraint on my schema devair from field user_id of table bluePrints to the PK id of table users but I get an error:
ERROR 1215: Cannot add foreign key constraint
Here are my table definitions:
users:
CREATE TABLE `users` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`email` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`password` varchar(60) COLLATE utf8_unicode_ci NOT NULL,
`remember_token` varchar(100) COLLATE utf8_unicode_ci DEFAULT NULL,
`created_at` timestamp NULL DEFAULT NULL,
`updated_at` timestamp NULL DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `users_email_unique` (`email`)
) ENGINE=InnoDB AUTO_INCREMENT=19 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
bluePrints:
CREATE TABLE `bluePrints` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`bluePrintName` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`description` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`user_id` int(10) NOT NULL,
`created_at` timestamp NULL DEFAULT NULL,
`updated_at` timestamp NULL DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `uid` (`user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
And the offending alter table statement:
ALTER TABLE `devair`.`bluePrints`
ADD CONSTRAINT `bp_u`
FOREIGN KEY (user_id)
REFERENCES `devair`.`users` (id)
ON DELETE CASCADE
ON UPDATE CASCADE;

Answer 1

The simple answer would be that you need to do this:
ALTER TABLE table1 ADD CONSTRAINT fk_bp_id FOREIGN KEY (columnFromTable1) REFERENCES table2(columnReferencedFromTable2);

Answer 2

The problem is the 'unsigned' in the bluePrints table. Once it's unsigned, then it works.
Now i have to figure out how to get laravel to use unsigned.